Poisson's ratio (ν), named after Simeon Poisson, is the ratio of the relative contraction strain, or transverse strain (normal to the applied load), divided by the relative extension strain, or axial strain (in the direction of the applied load). Some materials, called auxetic materials, have a negative Poisson's ratio (NPR). If such materials are stretched (or compressed) in one direction, they become thicker (or thinner) in perpendicular directions.
The vast majority of auxetic structures are polymer foams. U.S. Pat. No. 4,668,557, for example, discloses an open cell foam structure that has a negative Poisson's ratio. The structure can be created by triaxially compressing a conventional open-cell foam material and heating the compressed structure beyond the softening point to produce a permanent deformation in the structure of the material. The structure thus produced has cells whose ribs protrude into the cell resulting in unique properties for materials of this type.
Auxetic and NPR structures have been used in a variety of applications. According to U.S. Pat. No. 7,160,621, an automotive energy absorber comprises a plurality of auxetic structures wherein the auxetic structures are of size greater than about 1 mm. The article also comprises at least one cell boundary that is structurally coupled to the auxetic structures. The cell boundary is configured to resist a deformation of the auxetic structures.
NPR structures can react differently under applied loads. FIG. 1 illustrates a reactive shrinking mechanism, obtained through a topology optimization process. The unique property of this structure is that it will shrink in two directions if compressed in one direction. FIG. 1 illustrates that when the structure is under a compressive load on the top of the structure, more material is gathered together under the load so that the structure becomes stiffer and stronger in the local area to resist against the load.